The invention relates to presses for moulding plastics materials. More particularly the invention relates to such presses suitably but not exclusively for the manufacture of flat parts such as gramophone disc records. Presses for moulding plastics materials are known which comprise: a fixed press platen adapted to carry a first mould element, a movable press platen guided by parallel columns fast with the fixed press platen; a movable intermediate platen adapted to carry a second mould element; means capable of moving the movable press platen along the guide columns, between an operative position close to the fixed press platen and an inoperative position remote from the fixed press platen; a circuit for the admission of fluid under pressure into a chamber defined by a piston, formed by one of the movable platens, in a cylinder formed by the other of the movable platens; and a feed system for plastics material in the fluid state capable of introducing under pressure a given volume or metered amount of this plastics material in the fluid state between the two mould elements when the movable press platen occupies its operative position and when the aforesaid circuit delivers fluid under pressure into the aforesaid chamber. By feed system there is understood in particular injection systems (with plunger), intrusion systems (with rotary feed screw) and the like.
In the known moulding presses of the kind hereinbefore defined, the intermediate platen is guided by the columns and this complicates the machining of the movable platens and subjects the columns to bending stresses.
The invention has the object in particular of remedying these disadvantages.
In a moulding press according to the invention the movable platen and the intermediate platen cooperate one within the other as an arrangement of a piston in a cylinder with the intermediate platen supported primarily or wholly by the movable press platen. This considerably simplifies machining of the movable platens and relieves the columns of bending stresses, thus enabling these columns to have a lighter construction.
Moreover, in some known presses of this kind, the movable press platen and the intermediate platen are first brought up to the fixed press platen together so that a gap subsists between the two mould elements. Then the feed or injection system introduces the metered amount or charge of plastics material into the mould, which is partly open at the time by reason of the gap in question. Finally, the intermediate platen is shifted by a jack bearing on the movable press platen and which is then maintained in a fixed position, so as to apply the two mould elements one against the other and thus compress the charge of plastics material inside the mould.
These known presses have a certain number of drawbacks. In particular, the charge of plastics material is distributed badly in the partly open mould, in particular when the injected material is very fluid and more particularly still when a horizontal press is concerned, that is to say a press in which the plane of the joint between the mould elements is vertical. Moreover, the gases occluded in the injected material are liberated under conditions such that traces of bubbles or blisters may be left at the surface of the moulded parts, which is very specially inconvenient when it is a question of manufacturing gramophone discs, for which the state of the surface must comply with extremely rigorous conditions.